expose an optional Pool argument in most Future\'s combinators;

handlers of a future are scheduled in pool rather than sequentially (to avoid
possible deadlocks among them)

future.ml

@@ -25,25 +25,6 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 (** {1 Futures for concurrency} *)
 
-type 'a t = {
-  mutable content : 'a result;
-  mutable handlers : 'a handler list;   (* handlers *)
-  mutex : Mutex.t;
-  condition : Condition.t;
-} (** A future value of type 'a *)
-and 'a result =
-  | NotKnown
-  | Success of 'a
-  | Failure of exn
-  (** Result of a computation *)
-and 'a handler =
-  | OnSuccess of ('a -> unit)
-  | OnFailure of (exn -> unit)
-  | OnFinish of (unit -> unit)
-
-exception SendTwice
-  (** Exception raised when a future is evaluated several time *)
-
 (** {2 MVar: a zero-or-one element thread-safe box} *)
 
 module MVar = struct
@@ -268,11 +249,34 @@ end
 let default_pool = Pool.create ?timeout:None ~size:100
   (** Default pool of threads, should be ok for most uses. *)
 
+(** {2 Futures} *)
+
+type 'a t = {
+  mutable content : 'a result;
+  mutable handlers : 'a handler list;   (* handlers *)
+  pool : Pool.t;
+  mutex : Mutex.t;
+  condition : Condition.t;
+} (** A future value of type 'a *)
+and 'a result =
+  | NotKnown
+  | Success of 'a
+  | Failure of exn
+  (** Result of a computation *)
+and 'a handler =
+  | OnSuccess of ('a -> unit)
+  | OnFailure of (exn -> unit)
+  | OnFinish of (unit -> unit)
+
+exception SendTwice
+  (** Exception raised when a future is evaluated several time *)
+
 (** {2 Basic Future functions} *)
 
-let make () =
+let make pool =
   { content = NotKnown;
     handlers = [];
+    pool;
     mutex = Mutex.create ();
     condition = Condition.create ();
   }
@@ -305,8 +309,8 @@ let send future x =
     Condition.broadcast future.condition;
     List.iter
       (function
-        | OnSuccess f -> f x
+        | OnSuccess f -> Pool.run future.pool (fun () -> f x)
-        | OnFinish f -> f ()
+        | OnFinish f -> Pool.run future.pool (fun () -> f ())
         | OnFailure _ -> ())
       future.handlers;
     Mutex.unlock future.mutex
@@ -348,7 +352,7 @@ let on_success future k =
   (match future.content with
   | NotKnown ->
     future.handlers <- (OnSuccess k) :: future.handlers; (* wait *)
-  | Success x -> k x
+  | Success x -> Pool.run future.pool (fun () -> k x)
   | Failure _ -> ());
   Mutex.unlock future.mutex
 
@@ -358,7 +362,7 @@ let on_failure future k =
   | NotKnown ->
     future.handlers <- (OnFailure k) :: future.handlers; (* wait *)
   | Success _ -> ()
-  | Failure e -> k e);
+  | Failure e -> Pool.run future.pool (fun () -> k e));
   Mutex.unlock future.mutex
 
 let on_finish future k =
@@ -366,11 +370,12 @@ let on_finish future k =
   (match future.content with
   | NotKnown ->
     future.handlers <- (OnFinish k) :: future.handlers; (* wait *)
-  | Success _ | Failure _ -> k ());
+  | Success _ | Failure _ -> Pool.run future.pool (fun () -> k ()));
   Mutex.unlock future.mutex
 
-let flatMap f future =
+let flatMap ?pool f future =
-  let future' = make () in
+  let pool = match pool with | Some p -> p | None -> future.pool in
+  let future' = make pool in
   (* if [future] succeeds with [x], we spawn a new job to compute [f x] *)
   on_success future
     (fun x ->
@@ -384,14 +389,14 @@ let flatMap f future =
     (fun e -> fail future' e);
   future'
 
-let andThen future f =
+let andThen ?pool future f =
-  flatMap (fun _ -> f ()) future
+  flatMap ?pool (fun _ -> f ()) future
 
-let sequence futures =
+let sequence ?(pool=default_pool) futures =
   let a = Array.of_list futures in
   let n = Array.length a in
   let results = Array.make n NotKnown in
-  let future' = make () in
+  let future' = make default_pool in
   (* state: how many remain to finish *)
   let count = MVar.full (Array.length a) in
   (* when all futures returned, collect results for future' *)
@@ -425,8 +430,8 @@ let sequence futures =
   done;
   future'
 
-let choose futures =
+let choose ?(pool=default_pool) futures =
-  let future' = make () in
+  let future' = make default_pool in
   let one_finished = MVar.full false in
   (* handlers. The first handler to be called will update [one_finished]
      to true, see that it was false (hence know it is the first)
@@ -446,8 +451,8 @@ let choose futures =
     futures;
   future'
 
-let map f future =
+let map ?(pool=default_pool) f future =
-  let future' = make () in
+  let future' = make pool in
   on_success future (fun x -> let y = f x in send future' y);
   on_failure future (fun e -> fail future' e);
   future'
@@ -457,12 +462,13 @@ let map f future =
 let return x =
   { content = Success x;
     handlers = [];
+    pool = default_pool;
     mutex = Mutex.create ();
     condition = Condition.create ();
   }
 
 let spawn ?(pool=default_pool) f =
-  let future = make () in
+  let future = make pool in
   (* schedule computation *)
   Pool.run pool
     (fun () ->
@@ -508,6 +514,8 @@ let spawn_process ?(pool=default_pool) ?(stdin="") ~cmd =
         | Unix.WSTOPPED i -> i in
       (returncode, out', err'))
 
+(* TODO a global scheduler for timed events *)
+
 let sleep ?(pool=default_pool) time =
   spawn ~pool
     (fun () -> Thread.delay time; ())

future.mli

@@ -83,7 +83,7 @@ val default_pool : Pool.t
 
 (** {2 Basic low-level Future functions} *)
 
-val make : unit -> 'a t
+val make : Pool.t -> 'a t
   (** Create a future, representing a value that is not known yet. *)
 
 val get : 'a t -> 'a
@@ -111,19 +111,19 @@ val on_failure : _ t -> (exn -> unit) -> unit
 val on_finish : _ t -> (unit -> unit) -> unit
   (** Attach a handler to be called when the future is evaluated *)
 
-val flatMap : ('a -> 'b t) -> 'a t -> 'b t
+val flatMap : ?pool:Pool.t -> ('a -> 'b t) -> 'a t -> 'b t
   (** Monadic combination of futures *)
 
-val andThen : 'a t -> (unit -> 'b t) -> 'b t
+val andThen : ?pool:Pool.t -> 'a t -> (unit -> 'b t) -> 'b t
   (** Wait for the first future to succeed, then launch the second *)
 
-val sequence : 'a t list -> 'a list t
+val sequence : ?pool:Pool.t -> 'a t list -> 'a list t
   (** Future that waits for all previous sequences to terminate *)
 
-val choose : 'a t list -> 'a t
+val choose : ?pool:Pool.t -> 'a t list -> 'a t
   (** Choose among those futures (the first to terminate) *)
 
-val map : ('a -> 'b) -> 'a t -> 'b t
+val map : ?pool:Pool.t -> ('a -> 'b) -> 'a t -> 'b t
   (** Maps the value inside the future *)
 
 (** {2 Future constructors} *)